This error is popping up in our logs periodically every few seconds, and we have never seen it before last Friday and we can't reproduce it in our other environments (this is happening in our production environment)

Yes, judging from the stack trace, your server is trying to send messages to a queue that is no longer being used (no client is listening on it). This shouldn't happen. It seems some clients didn't reconnect properly after your server restart. Which version of Errai are you using? Also, are you operating a cluster or only a single node?

We have made fixes to the client-side reconnection logic in 3.0.2.Final. So, it's definitely worth upgrading (also because refreshes in development mode will be a lot faster).

Yes, the server keeps a queue for every connected client (associated with the HTTP session). From your stack trace it looks like your client had a valid HTTP session but no queue which is why I suggested the reconnection failed for some clients.

It wouldn't necessarily be strange for this to go on for a while as users could leave their browsers open and Errai will try to reconnect indefinitely. It's hard to tell though without digging deeper (look at the client logs, look at the messages that couldn't be sent, etc.).